Belt buckles generally have a load bearing frame attached to the vehicle floor or the like by means of a suitable attachment fitting, a locking mechanism arranged at the frame, and a housing which encloses the frame and which has an insertion slot for an insert tongue acting in conjunction with the locking mechanism. This insertion slot must be free and open so as not to obstruct the insertion of the tongue into the belt buckle when the safety belt is to be used. The open and unobstructed insertion slot, however, also allows the entry of extraneous matter into the inner space of the belt buckle. Examples of such extraneous matter are liquids (drinks), glass splinters (accident, break-in), fruit pips, rice grains, fibres, grit, gravel and the like. Hard matter, in particular, but also sticky liquids can impair or even impede the function of the locking mechanism. If proper locking is not achieved, the safety belt is not capable of fulfilling its protective function. Incomplete locking, also known as "pseudo-locking", is particularly dangerous, since the vehicle occupants are unable to detect this condition.